From Swedish Pat. No. 112 065 and the DE OS No. 26 45 141 there is previously known a projection screen structure comprising a rigid open box, the open side of which is closed by means of a flexible sheet which serves as a projection screen surface. By establishing a subatmospheric pressure within the box, the sheet is imparted a concave spherical shape.
Such structures, as well as concave spherical stiff projection boards are, however, bulky and not readily transportable from one place to another, especially when the effective screen surface thereof is to have dimensions of 1.5 by 2 meters or more.